There are scientists who believe that Mycobacterium Vaccae (a family of bacteria) whose natural habitat is in soil, may work as an anti-depressant because it stimulates serotonin and norepinephrine in the brain and helps to decrease anxiety levels.
More of this fascinating information can be found in The Atlantic :(http://www.theatlantic.com/health/archive/2012/01/how-to-get-high-on-soil/251935/)